Radio-frequency (RF) transmitters and receivers are found in a multiplicity of applications, in particular in the field of wireless communication and radar sensors. In the automotive sector, there is an increasing need for radar sensors which can be used, inter alia, in driving assistance systems (Advanced driver assistance systems, ADAS), for example in adaptive cruise control (ACC or Radar Cruise Control) systems. Such systems can automatically adapt the speed of an automobile in order to thus maintain a safe distance to other automobiles traveling in front (and from other objects and pedestrians). Further applications in the automotive sector are, for example, blind spot detection, lane change assist and the like.
Modern radar systems use highly integrated RF circuits which can contain all core functions of an RF front-end of a radar transceiver in a single chip housing (Single-Chip Transceiver). Such RF front-ends may have, inter alia, an RF local oscillator (LO), power amplifiers, low-noise amplifiers (LNA) or mixers.
Frequency-modulated continuous-wave (FMCW) radar systems use radar signals containing sequences of so-called chirps. In order to produce such chirps, the radar device may have a local oscillator which has a VCO arranged in a phase-locked loop (PLL). The frequency of the VCO is set using a control voltage which can be tuned by adjusting the frequency division ratio of a frequency divider in the feedback loop of the PLL. In order to keep the phase noise of the local oscillator output signal low, the bandwidth of the PLL should be low. However, a low bandwidth goes against generating highly linear chirp signals with steep frequency ramps.